The invention relates to a device for detecting the presence of a hard object inserted inside an envelope. The invention applies in particular to the automatic processing of mail items, such as postal sorting.
In the automatic processing of mail items, conveyor belts are generally used which define a relatively complicated conveyor path for the items, and which include several bends at 90.degree. and even at 180.degree..
The mail items are generally constituted by flat, flexible envelopes containing sheets of paper which are themselves flexible, such that the items curve easily around the bends in the path.
However, it often happens that some processed mail items contain a hard flat object such as a key, a metal plate, or the like. Unfortunately, the presence of that type of object in a mail item is likely not only to tear the envelope of the item, but also to cause the conveyor belts either to be damaged or to be jammed, in particular at the particularly sharp, above-indicated bends in the conveyor path.